Then, by slowly tilting the bottle even further (just make sure the bubble will not burst), we can see that the air bubble will also move down. At the end, we can use this method to "push" down the air bubble by quite a large distance, shown in the next image.
So, my question is what causes the air bubble to move downwards? My guess is that it is because of the surface tension of water. As the air bubble gets closer to the water level, the upper part will have a smaller surface tension compared to the lower part.
However, I am not entirely sure about this and I am not very familiar with the concept of surface tension. Is it caused by some other reasons or mechanisms I failed to realise?